Touch sensitive active matrix display and method for touch sensing

ABSTRACT

A touch sensitive active martix display device has an array of capacitive display element pixels ( 16 ), each associated with a pixel storage capacitor ( 20 ) and a pixel transistor. One or more common electrode contacts ( 18 a) are provided and connected to a terminal of a plurality of the display elements ( 16 ). Each common electrode contact is individually connectable to a charge measurement device ( 50 ) for measuring a flow of charge to the common electrode contact. The charge flowing through the capacitive display element can thus be measured while the pixel transistor is switched off. This flow of charge represents the transfer of charge between the pixel storage capacitor ( 20 ) and the display element ( 16 ) and results from a change in capacitance, and is therefore indicative of a touch input.

DESCRIPTION

This invention relates to active matrix liquid crystal displays, andparticularly such displays with a touch sensitive input function.

Active matrix displays typically comprise an array of pixels arranged inrows and columns. Each row of pixels shares a row conductor whichconnects to the gates of the thin film transistors of the pixels in therow. Each column of pixels shares a column conductor, to which pixeldrive signals are provided. The signal on the row conductor determineswhether the transistor is turned on or off, and when the transistor isturned on, by a high voltage pulse on the row conductor, a signal fromthe column conductor is allowed to pass on to an area of liquid crystalmaterial (or other capacitive display cell), thereby altering the lighttransmission characteristics of the material.

FIG. 1 shows a conventional pixel configuration for an active matrixliquid crystal display. The display is arranged as an array of pixels inrows and columns. Each row of pixels shares a common row conductor 10,and each column of pixels shares a common column conductor 12. Eachpixel comprises a thin film transistor 14 and a liquid crystal cell 16arranged in series between the column conductor 12 and a commonelectrode 18. The transistor 14 is switched on and off by a signalprovided on the row conductor 10. The row conductor 10 is thus connectedto the gate 14 a of each transistor 14 of the associated row of pixels.Each pixel additionally may comprise a storage capacitor 20 which isconnected at one end 22 to the next row electrode, to the preceding rowelectrode, or to a separate capacitor electrode. The capacitance of thepixel (capacitor 20 or self-capacitance) stores a drive voltage so thata signal is maintained across the liquid crystal cell 16 even after thetransistor 14 has been turned off.

In order to drive the liquid crystal cell 16 to a desired voltage toobtain a required gray level, an appropriate signal is provided on thecolumn conductor 12 in synchronism with a row address pulse on the rowconductor 10. This row address pulse turns on the thin film transistor14, thereby allowing the column conductor 12 to charge the liquidcrystal cell 16 to the desired voltage, and also to charge the storagecapacitor 20 to the same voltage. At the end of the row address pulse,the transistor 14 is turned off, and the storage capacitor 20 maintainsa voltage across the cell 16 when other rows are being addressed. Thestorage capacitor 20 reduces the effect of liquid crystal leakage andreduces the percentage variation in the pixel capacitance caused by thevoltage dependency of the liquid crystal cell capacitance.

The rows are addressed sequentially so that all rows are addressed inone frame period, and refreshed in subsequent frame periods.

As shown in FIG. 2, the row address signals are provided by row drivercircuitry 30, and the pixel drive signals are provided by column addresscircuitry 32, to the array 34 of display pixels.

The ability to interact with a display by using fingers (touch input) ora stylus (pen input) to allow input to the system connected to thedisplay is a highly desirable feature and a number of methods have beendeveloped to do this. In most cases, these methods involve the additionof extra components in front of, behind or around the edge of thedisplay.

It has been recognised that the liquid crystal layer of a display canalso be used as a pressure sensor. In particular, the application ofpressure to the liquid crystal layer changes the local electricalcapacitance of the layer, and this change can be used to detect thepresence of a pressure input at that point. Some schemes have beenproposed with simultaneous display and pressure sensing, and others havebeen proposed with sequential display and pressure sensing operations.

For example, JP 2000/066837 discloses a method by which the amount ofcharge required to recharge a pixel is measured and compared with thecharge required for other pixels. In this way, a change in capacitanceis detected, representative of pressure applied to the liquid crystalmaterial of the pixel. In U.S. Pat. No. 5,777,596, the charge time ofliquid crystal display elements are compared to a reference value inorder to determine which elements are being touched. When using chargetime or quantity as a measure of capacitance, the pixel needs to becompletely discharged, and charged to a given voltage, in order toenable a comparison to be made. This inevitably disrupts the normaldisplay operation. For example, in U.S. Pat. No. 5,777,596, a so-called“blinking line” approach is used. A blinking line progresses from thetop to the bottom of the screen, during which the display elements aredriven between fully discharged and charged states. Clearly thisprovides an undesirable image artifact. An alternative approachdisclosed in U.S. Pat. No. 5,777,596 is a so-called “hot spot cursor”approach, in which a smaller area is caused to blink, and this blinkingsmall area is dragged to the desired location. Again, the displayedimage is disturbed.

According to the invention, there is provided a touch sensitive displaydevice comprising an array of capacitive display element pixels, eachdisplay element being associated with a pixel circuit including a pixelstorage capacitor, each display element being connected at a firstterminal to the storage capacitor,

wherein the device further comprises one or more common electrodecontacts, the or each common electrode contact being connected to asecond terminal of a plurality of the display elements, and wherein eachcommon electrode contact is individually connectable to a chargemeasurement means for measuring a flow of charge to the common electrodecontact.

In this arrangement, the charge flowing through the capacitive displayelement to (or from) the second terminal can be measured. This flow ofcharge represents the transfer of charge between the pixel storagecapacitor and the display element, resulting from a change incapacitance of the capacitive display element, and therefore indicativeof a touch input. This charge measurement can be performed whilst thedisplay pixel is displaying an image and without changing the normaldisplay drive scheme.

There may be only one common electrode contact, which is the sharedcommon electrode contact for all pixels of the array. In this case,touch sensing resolution is achieved by means of row conductors.However, a plurality of common electrode contacts are preferablyprovided, so that resolution in row and column directions can beachieved. Each common electrode contact can then be connected to arespective charge sensitive amplifier (although a multiplexer could beused to enable an amplifier to be shared). The charge sensitiveamplifier preferably connects the common electrode contact to a virtualearth potential, so that the common electrode contact is held to ground.Thus, the provision of charge measurement does not affect the normaldisplay operation as the voltages used for the display operation arepreserved.

Preferably, the array of display element pixels is arranged in rows andcolumns, and wherein each common electrode contact is connected to thesecond terminals of the display elements of a plurality of adjacentcolumns of display elements pixels. The contacts thus provide resolutionacross the columns. Each row of display element pixels then shares acommon row conductor, and each pixel comprises a storage capacitorconnected between the display element and the row conductor of anadjacent row of display element pixels. This enables the charge flowingto the storage capacitor to be monitored by means of the row conductors,so that the combination of charge measurement for the columns and forthe rows allows the touch input location to be identified.

Preferably, a plurality of groups of adjacent rows are defined with eachgroup individually connectable to a charge measurement means formeasuring a flow of charge to the group of row conductors. This meansthat each touch input area is defined by the crossover of a group ofrows and columns, so that the sensitivity is improved.

The capacitive display elements may comprise liquid crystal displayelements.

The invention also provides a method of detecting a touch input in atouch sensitive display device, the device comprising an array ofcapacitive display element pixels each comprising a capacitive displayelement and a pixel storage capacitor, the method comprising:

applying display signals to the pixels of the array, by charging thedisplay element of each pixel to a desired voltage through a pixeltransistor;

isolating each pixel by switching off the pixel transistor, and storingthe voltage on the display element using the pixel storage capacitor;and

whilst the pixel is isolated, sensing the charge flowing between thestorage capacitor and the capacitive display element.

The sensing of the charge flowing enables a change in capacitance of thecapacitive display element to be detected, which is indicative of thedisplay being touched.

By sensing charge flowing after the pixels have been addressed, themethod avoids distortion of the displayed image to enable touch sensingto be implemented.

The sensing is preferably carried out by monitoring the charge flowingto a terminal of the capacitive display element. Preferably thisterminal of a plurality of display elements is monitored, the pluralityof display elements sharing a common contact and comprising a column orcolumns of display elements. Preferably, the charge flowing to aterminal of the pixel storage capacitor is also monitored. Preferably,the charge flowing to that terminal of a plurality of pixel storagecapacitors is monitored, the plurality of pixel storage capacitorssharing a common contact and comprising the pixel storage capacitors ofa row or rows of pixels.

Thus, row conductors and the common contact shared between columns ofthe display elements are monitored in order to enable the location oftouch input to be detected.

Changing display drive levels also results in capacitance changes, andtherefore charge flow. Thus, a subset of the pixels of the array may beused for touch sensing and display, the remaining pixels being used onlyfor display. In this case, substantially static images can be providedto the subset (for example alternate rows) of pixels.

Alternatively, the display data for the subset may be repeated, andtouch sensing is performed in the first or in a subsequent repetition,so that the image is then static. The subset may be different fordifferent frames, so that the area used for touch sensing moves aroundthe image. This enables the method to work reliably for moving images.

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a known AMLCD pixel;

FIG. 2 shows a known AMLCD display which may be modified in accordancewith the invention;

FIG. 3 is used to explain how an LC cell can be used for touch sensing;

FIG. 4 shows an equivalent circuit for an addressed pixel and is used toexplain the touch sensing operation in more detail;

FIG. 5 shows how display electrodes are arranged in accordance with theinvention; and

FIG. 6 shows a display in accordance with the invention.

It will be appreciated that the Figures are merely schematic. The samereference numbers are used throughout the Figures to denote the same, orsimilar, parts.

This invention provides a display and a drive method which allows thesensing of physical pressure caused by finger or a stylus on the frontof an LC display without adding extra components to the displaysubstrate. This is achieved by using components already present in thedisplay to do the sensing and connecting them to additional electroniccircuits. Furthermore, image distortion is avoided or kept to a minimum,and simultaneous display and sensing is obtained.

The basis of the system is to detect the change in capacitance of the LCpixels in the display caused by pressure on the front glass (orplastic). FIG. 3 shows a schematic cross-section of an active matrixliquid crystal display (AMLCD). The pixel capacitance is defined by thecapacitance between the pixel electrode 40 and the common electrode 18and is proportional to the reciprocal of the cell gap, 42. If pressureis applied, the substrates forming the LC cell can deform as can thespacer balls 44, causing a reduction in the cell gap, 42, and hence anincrease in the pixel capacitance.

The system of the invention senses the touch input during the periodwhen the TFT 14 of the pixel circuit (FIG. 1) is turned off, isolatingthe pixel and storage capacitors from the display columns 12. In thissituation the equivalent circuit of the pixel is as shown in FIG. 4. Thenode 23 is a node of the pixel circuit which is supplied with a pixeldrive voltage by the transistor 14. This circuit will be referred tofurther below.

FIG. 5 shows how the existing display components are used to provide thetouch sensing function.

The common electrode 18 is divided into separate contacts 18 a. Eachelectrode contact 18 a is connected to the second terminal of thedisplay elements of a number of columns of pixels. Each common electrodecontact 18 a is individually connectable to a charge sensitive amplifierfor measuring a flow of charge to the common electrode contact 18 a. Inthis way, the charge flowing through the LC cell 16 to the secondterminal (which is no longer common to all pixels) can be measured. Thisflow of charge represents the transfer of charge between the storagecapacitor 20 and the LC cell 16, and is indicative of a touch input.

The rows are also arranged in groups 10 a, so that touch sensitive inputareas 46 are defined by the crossover of a group 10 a of rows and agroup of columns sharing the common electrode contact 18 a.

If it is assumed that the common electrode is split into S segments andthe row conductors to which the storage capacitors are attached aredivided into R groups 10 a, this allows touch to be sensed in R×S areasof the display. Increasing R and/or S increases the resolution of thesensing but means that the size of the charge sensed will be smaller. Rand S can therefore be selected according to the demands of theapplication.

As there is a voltage V_(LC) typically in the range 2V-6V across the LCcell, an increase in the capacitance will cause a flow of charge intothe pixel from the common electrode 18, and from the connection to thestorage capacitor 20. This connection is an adjacent row in the pixelcircuit of FIG. 1, although it may be a separate capacitor line.

In the simple arrangement illustrated in FIG. 4, if the capacitanceC_(LC) of the LC cell 16 is changed by an amount ΔC then, provided ΔC issmall compared to C_(LC) and the storage capacitance C_(s) (of capacitor20), the amount of charge flow, ΔQ, required to hold the voltage acrossC_(s) and C_(LC) in series is given by:${\Delta\quad Q} = \frac{{\Delta C} \cdot V_{LC} \cdot C_{S}}{C_{LC} + C_{S}}$

The approximate magnitude of the charge displaced by touching thedisplay is easy to determine from this formula. For a typical AMLCD,C_(s) is approximately equal to C_(LC). The value of ΔC, assumingstandard cell thickness and dielectric constant for the LC material anda 5% change in cell gap, is 44 pf/cm². If V_(LC)=4V then ΔQ=88 pC/cm².If around 0.5 cm² of the area of the display is distorted then thecharge displaced will be around 45 pC which can easily be detected bystandard charge amplifiers connected as described below.

FIG. 6 shows a schematic diagram for a method of sensing the chargedisplacement produced by the distortion of the LC cell gap. Each group10 a of rows and each common electrode contact 18 a is connected to avirtual earth charge sensitive amplifier 50. A row group 10 a and acommon electrode contact 18 a are each illustrated as a single line inFIG. 6 for simplicity. When an area of the display is touched, thecharge will flow in one or more of the common electrode contacts 18 aand in one or more of the row groups 10 a. These charge flows will besensed by the charge sensitive amplifiers 50 connected to those rowgroups and common electrode contacts and will produce a change in thesignal on the output of the amplifiers. By continuously monitoring theoutputs of the S common electrode contacts and R row groups, the signalresulting from touching the display may be sensed and the position andarea being touched can be deduced by determining which amplifiers haveproduced the signal. The charge sensitive amplifiers are virtual earthamplifiers, so that the effects of cross coupling capacitances betweenthe common electrode contacts or the row groups in other parts of thedisplay are minimised.

The amplifiers also hold the row and column conductors to groundpotential during the touch sensing operation, which is compatible withthe normal display operation of the device.

A simple version of this system can be made with a single commonelectrode contact (S=1). This cannot detect horizontal position but candetect vertical position which, for many actions like selecting from astandard menu in which the items are all at different verticalpositions, gives all the information needed by the application.

In a standard LC display, changes in pixel capacitance can be induced bychanges in drive level on the pixels, since the LC dielectric constantand hence cell capacitance is drive-level dependant. This means thatchanging images can induce similar signals to those produced by a touchinput and could cause spurious touch detection signals to be generated.

For a static image there is no issue, and the sensing isstraightforward. Since, in the applications for which touch input isrequired, the images (for example of menus, key pads etc.) are usuallystatic, then the effect of changing images is not an issue. There isalso no problem if only a small fraction of the pixels in the area ofthe row blocks or common electrode segments change such that thecapacitance changes induced by the image changes are small compared tothose induced by touch pressure. Furthermore it is possible to allowlarger changes in image if some cause capacitance change in onedirection and some in the other, resulting in cancellation and zero orvery small change in overall capacitance. For example, an image withblocks flashing black to white and others of equal area under the samesegments flashing white to black (i.e. in antiphase) will not cause aproblem as the total capacitance will be constant.

It is, however, possible to adapt the use of the display to allow formoving images to be displayed whilst still enabling touch sensing.

The touch sensing is still carried out whilst the pixel is isolated,again sensing the charge flowing between the storage capacitor and thecapacitive display element. However, a subset of the pixels of the arraymay be used for touch sensing and display, the remaining pixels beingused only for display. In this way, substantially static images can beprovided to the subset of pixels. For example, only every other (orevery n_(th)) connection in the horizontal group of rows is used forsensing and the moving parts of the image are only directed onto therows which are not connected to the sense amplifiers. As a result, thereis no capacitance change in the sensing rows. This is more difficult toapply in the vertical direction and may only be applicable to imageswhere only selection of a vertical position is needed (as describedabove for the example where S=1).

Alternatively, the display data for the subset may be repeated, andtouch sensing is performed during the period when the image data isrepeated, so that the image is then static. Thus, the information on one(or more) of the sensing blocks can be intentionally repeated for 2 ormore frames to allow sensing of those blocks only. By shifting theposition of the active sensing blocks through the display from frame toframe, the entire display could be scanned for touch input. Theperceptive impact of this would be minimal.

In the example above, a terminal of each storage capacitor is connectedto the following row. Instead, additional capacitor contact rowconductors may be provided, and these will then be coupled to the chargesensitive amplifiers.

In this description, where the terms “row” and “column” are used, thisis purely arbitrary, and the display may be rotated by 90 degrees. Thus,these terms are not to be construed as limiting, and more significant isthat conductors cross (not necessarily at 90 degrees) in order to defineunique touch sensing areas.

The preferred implementation is for an LC display, but other capacitivedisplay elements, which display a change in capacitance in response toapplied pressure, could also be contemplated.

As explained above, the invention enables a normal drive scheme to beused for the display, although modifications are possible to ensure thatthe touch sensing is performed only in areas of the display where thereis no image change. The invention simply requires the addition of chargesensitive amplifiers, either in the row and column drivers (30,32 ofFIG. 2) or in additional dedicated circuitry. The invention alsorequires patterning of the common electrode layer—if more than onecommon electrode contact is desired.

Various other modifications will be apparent to those skilled in theart.

1. A touch sensitive display device comprising an array of capacitivedisplay element pixels, each display element being associated with apixel circuit including a pixel storage capacitor, each display elementbeing connected at a first terminal to the storage capacitor, whereinthe device further comprises one or more common electrode contacts, theor each common electrode contact being connected to a second terminal ofa plurality of the display elements, and wherein each common electrodecontact is individually connectable to a charge measurement means formeasuring a flow of charge to the common electrode contact.
 2. A deviceas claimed in claim 1, wherein a plurality of common electrode contactsare provided.
 3. A device as claimed in claim 2, wherein each commonelectrode contact is connected to a respective charge measurement means.4. A device as claimed in claim 1, wherein the or each chargemeasurement means comprises a charge sensitive amplifier.
 5. A device asclaimed in claim 4, wherein each charge sensitive amplifier connects thecommon electrode contact to a virtual earth potential.
 6. A device asclaimed in claim 1, wherein the array of display element pixels isarranged in rows and columns, and wherein each common electrode contactis connected to the second terminals of the display elements of aplurality of adjacent columns of display element pixels.
 7. A device asclaimed in claim 6, wherein each row of display element pixels shares acommon row conductor for providing a pixel address signal, and whereinthe storage capacitor of each pixel is connected between the displayelement and the row conductor of an adjacent row of display elementpixels.
 8. A device as claimed in claim 6, wherein each row of displayelement pixels shares a common capacitor row conductor, and the storagecapacitor of each pixel is connected between the display element and thecapacitor row conductor.
 9. A device as claimed in claim 7, wherein aplurality of groups of adjacent rows are defined with each groupindividually connectable to a charge measurement means for measuring aflow of charge to the group of row conductors.
 10. A device as claimedin claim 1, wherein each pixel circuit comprises a transistor which isaddressed by a signal on a row conductor associated with a row ofdisplay element pixels, and which provides a signal from a columnconductor associated with a column of display element pixels to thedisplay element.
 11. A device as claimed in claim 1, wherein thecapacitive display elements comprise liquid crystal display elements.12. A method of detecting a touch input in a touch sensitive displaydevice, the device comprising an array of capacitive display elementpixels each comprising a capacitive display element and a pixel storagecapacitor, the method comprising: applying display signals to the pixelsof the array, by charging the display element of each pixel to a desiredvoltage through a pixel transistor; isolating each pixel by switchingoff the pixel transistor, and storing the voltage on the display elementusing the pixel storage capacitor; and whilst the pixel is isolated,sensing the charge flowing between the storage capacitor and thecapacitive display element.
 13. A method as claimed in claim 12, whereinthe sensing is carried out by monitoring the charge flowing to aterminal of the capacitive display element.
 14. A method as claimed inclaim 13, wherein the charge flowing to a terminal of a plurality ofdisplay elements is monitored, the plurality of display elements sharinga common contact and comprising a column or columns of display elements.15. A method as claimed in claim 14, wherein the sensing is carried outby also monitoring the charge flowing to a terminal of the pixel storagecapacitor.
 16. A method as claimed in claim 15, wherein the chargeflowing to a terminal of a plurality of pixel storage capacitors ismonitored, the plurality of pixel storage capacitors sharing a commoncontact and comprising the pixel storage capacitors of a row or rows ofpixels.
 17. A method as claimed in claim 12, wherein a subset of thepixels of the array are used for touch sensing and display, theremaining pixels being used only for display.
 18. A method as claimed inclaim 17, wherein substantially static images are provided to the subsetof pixels.
 19. A method as claimed inclaim 17, wherein the subsetcomprises a plurality of rows of pixels.
 20. A method as claimed inclaim 17, wherein the display data for the subset is repeated, and touchsensing is performed in the first or in a subsequent repetition.
 21. Amethod as claimed in claim 20, wherein the subset is different fordifferent frames.